Hydrogen-induced cracking (HIC) can be a major problem for pipelines and pressure vessels, specifically those that service wet sour conditions, i.e., operate in a wet H2S-containing environment. In general, HIC is a bulk type of cracking in metallic structures (e.g., steel), such as pipelines, piping systems and pressure vessels, that can occur as a result of atomic hydrogen being dissolved in the metal. In particular, hydrogen atoms, which are either produced (1) as a result of the corrosion reaction between H2S and iron taking place at the metal surface, or (2) by poor cathodic protection (overprotection), can diffuse through interstitial sites in the metal, and recombine to form high-pressure hydrogen gas within the metal imperfections. The increased pressure of the hydrogen gas within the metal defects then causes cracks or blisters to form and grow in the bulk metal, which can subsequently link to each other in a stepwise manner and may lead to the structural failure of the metallic structure. Structural failure within sour service pressure equipment can results in safety and environmental hazards due to the potential leaking of sour gas. As such, the ability to predict and track the initiation and growth of HIC in sour service equipment is of the utmost importance.
Currently, sour service operations regularly monitor the equipment for signs of HIC, and when HIC is discovered, the affected locations are inspected even more frequently to determine whether pressure de-rating is required and ultimately when replacement equipment is needed. For example, for sour service systems, all vessels with linear HIC damage can be monitored by advanced ultrasonic testing on a regular interval (e.g., a yearly basis), while vessels showing step-wise cracking damage (a more severe form of HIC) can be monitored more frequently (e.g., semi-annually). This frequent monitoring, however, is costly and time-consuming. For a given metal grade (e.g., steel grade), knowledge of the HIC growth rate and its relationship with the operating conditions (e.g., temperature, pressure, pH, percentage of H2S) would allow for greater efficiency in the monitoring of HIC-damaged vessels. In particular, with a greater understanding of the factors controlling the growth rates of HIC, monitoring could be limited to those vessels at the highest risk of failure, rather than monitoring all HIC-damaged equipment on a systematic basis. In other words, the monitoring procedures could move from a schedule-based inspection (SBI) system to a risk-based inspection (RBI) system. As such, there is a need for reliable ways to predict the initiation and quantify the growth rate of HIC damage in metallic structures operating in a sour service environment in order to enhance the efficiency of equipment monitoring procedures.